| Since their naissance, white light emitting diodes (LEDs) are expected to be a good candidate as a promising general lighting source substituting for incandescent lamps or fluorescent lamps, resulting from their excellent properties, such as high brightness, long lifetime, low applied voltage, and high power efficiency. Traditionally, white LEDs are prepared by a combination of the blue emitting chip and the yellow Ce:YAG phosphor placed around the chip surface using epoxy resin or silicone. However, with improvement of the energy density in case of high power white LEDs, the resin-phosphor or silicone-phosphor converter cannot be used anymore because of the deterioration of the epoxy resin and silicone matrix (browning). In addition, the difference of refractive index between phosphor and encapsulation polymer material can cause light scattering. These may cause some adverse effects on the performance white LEDs, such as degradation of quantum efficiency and long-term reliability, shifts in emission color, and decrease of lifetime.To solve these problems, several fluorescence of bulk materials had used, such as single, ceramic and glass, to replace the resin-phosphor or silicone-phosphor converter. Due to its excellent thermal stability, stable chemical properties and unique homogenous optical properties, transparent yttrium aluminum garnet (Y3Al5O12, YAG) polystalline ceramic has been proved to be an attractive substitute material for LEDs application. Compared with YAG single crystal, the fabrication of transparent YAG ceramic is lower cost and can provide not only flexibility in size and shape design, but also a homogeneous distribution and a higher dope concentration of rare-earth ions. In addition, transparent YAG cermaics exhibit excellent optical, high thermal stability, and menchanical properties, which is better than glass. Furthermore, light scattering can be freely controlled by the processing and sintering route of ceramics. Therefore, ceramic phosphors have the potential substitute the silicone-phosphor converter in high power white LEDs.in this work, spherical YAG:Ce particles were successfully synthesized at 350? by the molten-salt method. The effect of temperature and amount of salt on the crystallization and particle size of YAG:Ce were investigated thoroughly. The results demonstrated that the powders prepared at 350? in salt to reactant ratio 2:1 were pure YAG:Ce phase with 200-300 nm in particle size. The as-synthesized phosphors were later post annealed at 1200? in nitrogen, air and oxygen atmospheres, respectively. The results showed that the emission intensity of YAG:Ce sensitively depended on the post-treated atmosphere and the phosphor annealed in nitrogen showed the highest emission intensities and a good thermal stability. However, the YAG powder prepared by MSS had not been successfully sintered to be transparent by vacuum sintering or spark plasma sintering, resulting from its hollow sphere morphology.Besides, commercial ?-Al2O3 and Y2O3 powders with 0.5% TEOS were used to fabricate transparent polycrystalline YAG ceramics by reactive spark plasma sintering. The powders mixture were dispersed by adjusting pH and the addition of PAA and citric acid in aqueous slurry. Single-phase YAG specimens sintered at 1400? for 3min were used to observe the microstructure and the optical properties. It was found that a pretreatment of the powders mixture by 0.6wt% citric acid significantly improved the microstructure homogeneity of the as-sintered specimen. After post annealed in wet air, the specimen sintered from the treated powders mixture was transparent, while the specimens from the untreated powders mixture was hazy. It is clear that citric acid had little influence on the particle size and size distribution in mixture slurry. Instead, citric acid might be strongly adsorbed in the surface of Y2O3 to prohibit serious hydrolysis reaction and thus lead to a good homogeneity in the final Al2O3+Y2O3 mixture, which can promise a complete chemical reaction for form high purity YAG phase. |
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